In general, the energy level of the emitted X-rays from an X-ray machine varies directly with the high voltage electrical potential across the X-ray tube. The usual practice is to rate the X-ray machine output in terms of the maximum applied potential, or kilovolt peak (kVp), which corresponds to the highest energy X-ray photons emitted from the source, since the highest energy X-ray photons determine the effective penetrating power of the X-ray beam.
Because of occasional calibration errors and changes in operating characteristics over time, the actual kVp applied to the X-ray tube may not be equal to the kVp setting indicated on the control panel of the X-ray machine. It is thus normal practice to periodically check the accuracy of the kVp setting on X-ray machines and recalibrate if necessary. This may be done directly by measuring the voltage applied across the X-ray tube, but because of the practical difficulties and hazards of measuring the high voltage applied to the tube it is often preferable to use an indirect method of measurement which deduces the kVp level by taking measurements of the radiated X-ray beam.
One type of device presently available for determining the kVp level from X-ray source beam measurements utilizes a pair of X-ray sensitive detectors which are located behind X-ray absorber filters of different thicknesses or materials. The kVp level of the X-ray tube can be determined from the ratio of the output signals from the two sensors, which signals are related to the radiation intensities incident upon the sensors. The sensors, typically semiconductor photodiodes, are generally placed closely adjacent to one another to sample essentially the same portion of the X-ray beam. However, if the X-ray intensity distribution in the beam varies from the position of one sensor to the other, erroneous readings will be obtained by the devices. Moreover, the readings obtained depend on the proper orientation of the sensors with respect to the beam, and consistent readings will not be obtained if the device is not precisely and consistently aligned with the midline of the beam. For example, if the two adjacent sensors are lined up in one orientation facing the X-ray tube for one test exposure and are then lined up in an orientation rotated 90.degree. in the plane facing the X-ray tube for the next exposure, the kVp readings obtained from the device for the two exposures will often not be the same.
Existing instruments of this type also tend to be sensitive to the absolute intensity level of the X-ray beam and may require adjustment by the operator prior to the taking of an exposure to accommodate the device to an expected X-ray intensity range. However, if the actual output from the X-ray machine is above or below the anticipated range, the readings from the device will be in error and further exposures must be taken.